Effect Of Pre-aging On Microstructure And Properties Of Hot Extruded Mg-Zn-Gd-Er Alloy

In this paper,a small amount of rare earth elements(wt.%=1)was added to the magnesium alloy to introduce lattice defects to accelerate and enhance the aging process.The pre-aging treatment and hot extrusion process were combined to improve the properties of magnesium alloy by the interaction between the twinning behavior and recrystallization of the second phase particles during the extrusion process.By means of OM,SEM,EDS,XRD and EBSD,the evolution of the second phase particles and its effect on the mechanical properties of Mg-6Zn-1Gd-1Er(wt.%)alloy during the aging process before hot extrusion were studied.The influence mechanism of the second phase particles with different morphology,size and volume fraction on twinning behavior,recrystallization behavior and texture during extrusion.The results show that:(1)The microstructure of as-cast Mg-6Zn-1Gd-1Er alloy is mainly composed of?-Mg matrix,Zn-rich phase and eutectic phase Mg₅(Zn,Gd,Er).The average grain size of the as-cast Mg-6Zn-1Gd-1Er alloy is 60?m.After T6 treatment,the eutectic phases in the as-cast microstructure were completely dissolved and the hardness increased by about 37%,which was attributed to the high density of the W and I phases.At the beginning of aging,the W phase is spherical.In the middle of aging,phase W gradually evolves into a"slab-like"morphology,accompanied by phase I precipitation.At the later aging stage,the W phase grows further and the size is about 25?m.The peak aged alloy shows better comprehensive mechanical properties,with tensile strength of 190 MPa,yield strength of 122 MPa and elongation of 10.3%.(2)The size of most of the second phase particles in the peak aged extrusion sample is0.3?1?m,the spacing is 2?5?m and the"spherical"phase is distributed within the grain,which can more effectively excite the grain nucleation(PSN)and refine the grain.Such particles distributed in grain boundaries will make the grain boundaries appear"concave-convex"morphology,providing more nucleation sites for recrystallized grains,thus refining the grains.However,in the solid solution state,the volume fraction of the second phase particles is small,and the contribution of particle nucleation is small.The higher the volume fraction of the second phase preset with appropriate size and spacing by aging treatment,the greater the contribution of particle nucleation.(3)In the initial stage of extrusion,{10-12}tensile twins are found in the alloy.Due to the higher volume fraction of the second phase particles in the aging state,more nucleation points are provided for the twinning,which promotes the twinning induced recrystallization and speeds up the process of dynamic recrystallization.The texture evolution of Mg-6Zn-1Gd-1Er(wt.%)alloy with different morphology did not change during the extrusion process,and the final microstructure formed the non-fiber texture characteristics of uniform distribution along the arc between[10-10]//ED and[2-1-10]//ED.However,the texture strength of the aging extruded sample shows a tendency of aggregation towards{2-1-10},which is related to the degree of particle nucleation and is controlled by the morphology,size and volume fraction of the second phase particles.In the peak aging extrusion state,the volume fraction of the second phase particles is the largest,the contribution of twin-induced recrystallization and particle excitation nucleation is the highest,and the best comprehensive mechanical properties are shown:the tensile strength is 346 MPa,the yield strength is 217 MPa,and the elongation is13.6%.(4)Under the action of shear stress,the high dislocation density and vacancy concentration are formed in the solid solution extruded sample,which leads to the dynamic precipitation of the second phase particles.During shear deformation,the free energy difference between precipitate and matrix,temperature rise and strain rate can promote the dissolution of pre-aged phase.